1. Field of the Invention
The present invention relates generally to electronic amplifiers, and more specifically to an operational transconductance amplifier.
2. Description of the Prior Art
An operational transconductance amplifier (OTA) is similar to the well known operational amplifier. It differs in that it consists of only one stage, and is exclusively intended for use with capacitive loads.
One design for an OTA includes a differential input stage, and four current mirrors. An example of such a circuit is shown in FIG. 1. In order to obtain a common mode input range which extends from the negative to the positive supply voltages, complementary pairs of input transistors must be used in the differential input stage. When the common mode is in the mid-range, both transistors of each pair are on. Near the positive or negative supply, only one transistor pair is on.
This means that the transconductance of the input stage, and therefore the overall amplifier, is not constant over the full common mode input voltage range. A differing transconductance over this range causes a different gain-bandwidth product over the input range. Additionally, the slew rate of the overall amplifier changes with such changing transconductance. These effects negatively impact the low supply performance of the device.
It would therefore be desirable to provide an input stage for an amplifier which has a constant transconductance over the full common mode voltage range.